This invention relates to membrane separation processes, such as ultrafiltration and reverse osmosis.
Ultrafiltration and reverse osmosis are similarly characterized by the function of separating solutions into solvents and concentrated solutions by passing the starting feed solutions in contact with membrane surfaces under pressure to permit only the solvents to permeate through the membrane.
The advantages of these membrane separation processes include the following: Since these processes can be performed without any change of phase, (1) they are less expensive in running cost and higher in energy efficiency than the distillation process, the crystallization (or freezing) process, and the electrodialysis process; (2) they are advantageous for separating and purifying thermally unstable materials and biologically active substances; and (3) they are excellent for recovering valuable materials from dilute solutions.
However, there are significant differences between them in the following respects, due to differences in their transport mechanism: (1) the molecular weight of solutes in the solutions applicable to ultrafiltration is several tens or more times that of the solvents (usually water) used, but in reverse osmosis the molecular weight of the solutes does not exceed about twenty times that of the solvents (usually water); (2) the applied pressure of the starting solution is usually about 0.5 to 10 kg/cm.sup.2 in ultrafiltration, but is about 10 to 100 kg/cm.sup.2 in reverse osmosis.
In the membrane separation processes, membrane performance is an important factor. The term "performance" in this context has reference to such properties as water flux, permselectivity, and their stability the water flux and the permselectivity for long term operation, the stability depending upon the physical and chemical stability of the membrane materials, and upon their resistance to fouling.
Poly (arylether-sulfone) is an excellent membrane material in terms of physical and chemical stability, and Amicon Corp. has obtained patents for ultrafiltration membranes from poly (arylether-sulfone) as described in U.S. Pat. Nos. 3,556,305; 3,615,024; 3,567,810, Brit. Pat. No. 1,238,180, etc. Further, Rhone-Poulenc S. A. has obtained patents for a method of preparing reverse osmosis membranes by introducing sulfonic groups into the poly (arylether-sulfone) as described in Japanese patent Application Laid-Open No. 852/1973 and 853/1973.
However, the poly (arylether-sulfone) has only limited solubility in non-aqueous polar solvents, such as N,N-dimethyl formamide, dimethyl sulfoxide, N,N-dimethyl acetamide, N-methyl-2-pyrrolidone, and hexamethylphosphoramide. Thus, membranes obtained from casting solutions by using these solvents are inclined to clog easily, so that their performance is unsatisfactory in long-term operation. In contrast to this, the modified poly (arylether-sulfone) in this invention, wherein chloromethyl groups and/or quaternized nitrogen atom-bearing groups are introduced into the poly (arylether-sulfone), has improved solubility in the foregoing polar solvents. As a result, the membranes prepared from the modified poly (arylether-sulfone) are more resistant to clogging and are satisfactory in long-term operation.
The membranes of poly (arylether-sulfone) containing introduced sulfonic groups have reduced chemical stability when they are used at a high pH above 12. The new membranes we have invented, however, possess excellent chemical stability even when used at a high pH about 12.
The influence of substituent groups on permselectivity depends on the nature of the introduced groups. While the rejection (% of solute substance that does not pass through the membrane) of the membranes made from sulfonic group-containing polyphenylene oxide increases in the order of MgCl.sub.2 &lt;NaCl&lt;MgSO.sub.4 &lt;NaSO.sub.4 as described in "Reverse Osmosis Membrane Research", Plenum Press, N. Y., 1972, edited by Lonsdal & Podall, the rejection of membranes prepared from quaternized nitrogen atom-containing poly (arylether-sulfone) increases in the order of Na.sub.2 SO.sub.4 &lt;MgSO.sub.4 &lt;NaCl&lt;MgCl.sub.2. These facts indicate that the performance evidently varies with the type of the substituent groups.